Hardness testing block



FIG. 1 is a top plan view of a hardness testing block showing my new design;

FIG. 2 is a rear elevational view thereof;

FIG. 3 is a left side elevational view thereof;

FIG. 4 is a bottom plan view thereof;

FIG. 5 is a top right-front perspective view thereof. 

The ornamental design for a hardness testing block, as shown. 